Electrostatic printing



April 11, 1961 Filed Oct. 24, 1958 E. c. GlAlMo, JR

ELECTROSTATIC PRINTING 24 fwn' W424i (anw/VAL) Iglu.

:ssa-2.- Z, DEVELOP 2 Sheets-Shea?l 1 INVENTOR. EDWARD E. Emma, JR.

April l1, 1961 E. c. GlAlMo, .1R 2,979,403

ELEcTRosTATIc PRINTING Filed oct. 24, 1958 2 sheets-sheet 2 bil/flap F/Pfr :W4/Pif (apr/MAL) INVENTOR. EDWARD E. EmmaJ IR lstatic latent images of negative polarity.

sible through use of a negatively charged powder in the f "developer mixture to produce reverse development of 'the electrostatic latent image. Y

United States atent ELECTROSTATIC rRNriNe Edward C. Giaimo, Jr., Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed Oct. 24, 1958, Ser. No. 769,494

7 Claims. (Cl. 9o-4l) ln electrostatic printing it is usual to form a latent electrostatic image by applying an electrostatic charge to the surface of a photosensitive member and selectively dissipating the charge by exposure to a pattern of light and shadow to be recorded. Thus, in `one system it is conventional to place a negative polarity electric charge on the surface lof a photo-conductor and to form an electrostatic image by exposing the charged surface, optionally through a suitable projection system, to a light image of a document, scene, or other image to be reproduced. The resulting electrostatioimage is thereupon developed by dusting it with an electroscopic developer powder which adheres to the charged portions of' the surface. In this manner, there is formed a reproduction of the original being photographed or copied.

By the operation just described, it is apparent that there is formed a dust image corresponding to a direct reproduction of the original. Thus, the portions of the original that are white or light in tone are reproduced by the absence of deposited powder, whereas the dark portions of the original are reproduced by a heavy powder deposit. Occasionally, however, it is desired to employ electrostatic printing for the production of reverse or negative images in which the original being produced in itself corresponds to a tone reversal of an original scene or document. For example, if it is desired to make a positive electrostatic print from a conventional photograph negative, it is necessary to reverse the blacks and whites.

According to one present method of electrostatic printing a reverse image is most suitably formed by use of special developer powders. According to this procedure a developer powder consisting of relatively larger carrier particles carrying on their surface line developer powder particles, is cascaded across the image surface. By proper selection of the surface electrostatic properties of the carrier particles and the developer powder, it is possible to produce a developer which, if desired, may contain posi- Vtively or negatively charged powder pmticles.

Conventional methods of electrostatic printing produce electrolt is then pos- A positively charged de- Veloper-powder produces a direct image.

A disadvantage of this system of reverse electrostatic printing isthat *one ytype of developer powder is required forrforming a direct image and a diiferent type of develyoper'powder yis required for forming a reverse image. `Since the process of electrostatic printing heretofore described lends itself particularly Well to rapid'and continuous machine printing processes, it would be desirable to iind a system wherein av reverse image might be provided with minimum additional equipment complexity. Use of a single developer powder for both direct and reverse image production Would be highly desirable.

An object of the invention is to provide improved methods of and means for electrostatic printing for producing a reverse visible image from a latent electrostatic charge image.

Another object is to provide improved methods of and means for obtaining a reverse visible image with the same developing powder used for obtaining a direct image from a latent electrostatic charge image.

A further object is to provide an improved method of and means for reversing the charge pattern defining an electrostatic latent image whereby the image may be directly developed to form an electrostatic print corresponding to a photographic reversal of the original exposure.

in general, the objects of the present invention may be accomplished by first forming an electrostatic latent image by usual well known methods and, before applying developer powder, recharging the latent electrostatic image and briefly exposing the surface to a source of uniform illumination. By this means the image configuration is maintained and the charged and uncharged areas are reversed. A reverse visible image is obtained upon developing.

The invention is described in greater detail by reference to the accompanying drawings wherein similar reference characters are applied to similar elements, and in which:

Figures la to lf are partially sectional views `of a first apparatus illustrating the steps for carrying out the improved processes of the invention; and,

Figure 2 is a partially sectional view of a second apparatus for carrying out the improved processes of the invention.

Referring to Figures la to 1f, in accordance with the invention a suitable photosensitive member 20 including a .photoconducting insulating layer or surface 22 is sensitized by charging' the surface, or in other words, depositing electrostatic charge on the photoconductive surface. The phctosensitive member preferably is placed on a grounded metal backing plate 24 during charging. The photosensitive member may comprise a paper backing 26 coated with zinc oxide in a silicone resin, and is sensitized immediately prior to use by passing across the surface a corona discharge electrode 28 adapted to deposit on the surface negative charge in the order *of several hundred volts. The corona discharge member comprises a plurality of three mil corona discharge wires 29, disposed together and spaced about 0.5 inch from each other and from the photoconducting coating 22. The corona `discharge wires are connected to a negative voltage source V (not shown). As an example, a negative voltage of 6000 volts produces a corona discharge which will deposit a negative electrostatic charge upon the surface of the photoconducting coating 22. As the photoconducting coating 22 passes beneath the wires '29, a corona discharge issuing therefrom is attracted to the grounded metal backing plate 24 placed behind the paper backing 26 and deposits upon the surface of the coating 22.

The electrostatically-charged or sensitized photoconductive coating 22 is then exposed to an incident electromagnetic radiation irnage, for example, alight image from a projector 30. The electrostatic charge is lost or reduced in the illuminated areas` and retained in the nonilluminated areas to form a latent electrostatic charge image on the surface 22. in prior art operation the photosensitive surface 22 after exposure bears an electrostatic latent image which is passed directly into a development operation. According to the present invention -thereto. l l s s an electric resistance element 44 connected to a voltage 3 'i' however, the photosensitive sheet bearing the latent electrostatic image lis passed through two additional operations by means of which a reverse visible image is obtained. After the photosensitive member is exposed ,to the image, it is again sensitized by depositing a second negative,electrostaticrcharge on the surface of the member, as in the previousoperation. The photosensitive 'sheet is then exposed very briefly to a'source of uniform illumination, from a light bulb 34 for example.

The result of these operations is an electrostatic latent image containing neutral areas, or areas of substantially low negative charge,v corresponding yto lthe areas which had previously been highly charged, and areas, of substantially high negative charge correspondingr to areas which hadppreviously been discharged. Thus, if a negatively. charged photosensitive member has been exposed to a photographic negative, the transparent areas in the photographic negative Vbecome highly charged to a nega- -tives'poplarity in the Anal latent image and the portions of the final latent imagecorresponding to dark areas in the photographic negative become substantially neutral. This vreverse electrostaticflatent image is ,then passed toV a development operation in which triboelectrically posi- `tive developer powder 36 is applied to the image surface -22 by applying sufficient heat to fuse the powder yimage to the coating, thereby Afixing the developed image 42 A source' ofV radiant heat may-be provided by source 46.V v y Although not intending to be limited thereby, a theory of operation for the described series of operations is proposed. When the surface of the photosens'itive layer is initially charged negatively, a corresponding positive -charge appears at the interface of the photosensitive layer higher :electrostatic field-'is' retained in the nonilluminated -areas Vsurface chargeis againapplied to the photosensitive layer.

During the second chargingroperati'on a uniform The 'non-illuminated area then assumes va higher negative charge than the illuminated area due bothl to the charge remaining after exposure tothe image and to the fact that the Vcharge cannot be raised to ashigh a value in theilluminated areas `due to slow recovery of the resistivity ofthe photosensitive material. The flashof unilform illumination then causes the areas ofhighest charge ythose areas which were not illuminated upon exposure to Yfthe image to recombine more rapidly than those areas with the lower charge, to form a latent image in which the previously illuminated areas have a higher electro static charge than in theY non-illuminated areas.' On

developmentV with triboelectrically kpositive developer powder areversevisible image results. v ,i

It is also knownl that 'in the conventional method of forming a latent electrostatic chargeimage it is possible to omit the step of first applyingi'an electrostatic charge to the surface of a photoconductive,insulating material.

A somewhatrlosnger exposure to an imageis.thenrequi'recL but this is compensated for by permittingelimination ofV one lof the charging steps. The mechanism of this opera- 4 t y It has been found that the methods of the present invention may be employed with photoconductors wherein the initial surface charging step is omitted, and reverse visible images may still be obtained.

' Figure 2 shows another apparatus utilizing the method of the invention, and comprises a paper backing 26 having a photoconductive coating 22 on one surface thereof passing between and over pulleys 48 and 50. A series of stations for producing reverse visible images by an electrostatic printing process are disposed along the path of the paper and between the pulleys 48 and 50.

At one station the photoconducting coating 22 is provided with a first uniform electrostatic charge on the surface thereof by means of a plurality of corona discharge wires 52 and a metal backing plate 54. The corona charging mechanism is of the same type as heretofore described.

The electrostatically-charged photoconductive coating 22 now passesto a station where it is exposed to an incident electromagnetic radiation image, for example, a light image from a projector 56. The electromagnetic radiation may be derived by any of the methods well known inthe photographic art, for example, by projection from a photographic transparency. Upon exposure to the electromagnetic radiation image, the illuminated areas of theV photoconducting coating 22 discharge the charge stored onthe surface thereof leaving the nonilluminated areas charged. This produces a first latent electrostatic image substantially corresponding to the electromagnetic radiation image.

YThe photoconducting coating 22 now passes to a station where it receives a second uniform electrostatic charge described. Y

tion where' it is briefly :exposed to a source of uniform iillumination from a light source 57 in a shielded enclosure The photoconducting 'coating 22 now lpasses to a sta- 58, thereby providing a reverse latent electrostatic image. The illumination 'for this `purpose is most conveniently derived from an incandescent lamp, 'and is chosen to be of the type to which the photoconducting coating 22 is sensitive,

The photoconducting coating 22 now passes to'a station where the reverse latent electrostaticimage is developed to a visible image according to'any of several ,rotary magnetic pole piece 62. A magnetic field is main- .tained Vtherebetween throughl-an enclosed magnetic field 'pathandacross the gap therebetween through which the photoconductive sheet passes.V A trough 66 holds a quantity of a developermix vwhich comprises developer powderparticles and magnetic carrier particles in' contact with the discs 62. When the Vdiscs are rotated in a clockwise position, as .viewed in Figurei2, a quantity 68 ofideveloper mix is carried upwardly and swept across ,the surface of the photoconductingcoating 22. kDe` .veloper powder particlesA are deposited 1in selected areas .thereby producing a ldeveloped powder image 70.on the ls urface ofthe photoconducting coating 22 corresponding e to theY reverse latent electrostatic image.

`The photoconductive Acoating 22 nowrpass'es toa sta- ,tionwhere the developed image is fixed to the coating. Y For this purposean electric resistance element 72 is conlnectedwto avoltage sourceV-V (not shown) which radiates heatzsufce'ntto fuse the powder Vimage 70 to y the photoconductive coatingl 22,r fixing; the developed g image 74 thereto..- Other methodsof fixing such as spray-V ,ing with an adhesive oursolvent vapors and/or pressure tion is not completely understoodat. this time, butV is believedptorbe associated with'` charge carriers located in discrete statesA in the forbidden band of'asemiconductmf. 75V ductive surface toa'so'urcie of uniform illumination areA Arnay'bensed."YV ,Y .7 y g s* c, 4In* the Ysystenrshown'gin' Figure the second charging 'step ,and the step ofy briefly` lexposing the` photoconomitted, as by deenergizing these stations, an electrostatic print corresponding to an original is obtained. By energizing these two stations, a reverse print is obtained.

As discussed heretofore, it is also possible to omit the station in which the first charge is applied to the photoconductive insulating coating 22. When this is done, it is necessary to use a longer image exposure time.

The methods heretofore described may be employed with paper coated with a photoconductive composition of zinc oxide in a silicone resin and also coated with zinc oxide in a silicone resin sensitized with certain dyes such as Rose Bengal or fluorescein. Other sensitizing materials may also be used, but best results have thus far been obtained with Rose Bengal since zinc oxide so sensitized has exhibited the greatest change in resistivity upon application of charge and exposure to light.

One example of a suitable coating composition may be prepared by intimately mixing 100 grams of a French process type white photoconducting zinc oxide with 65 grams of a 60% solution of a silicone resin in xylene (such as GE. SR-S2 marketed by the General Electric Co., Silicone Products Division, Waterford, New York), and 85 grams of toluene. To these materials about .07 gram of Rose Bengal is added. Incorporation of the Rose Bengal presents no processing problems as it is merely added during dispersion of the zinc oxide in the resin solution. Additional data on dye sensitization is described by C. I. Young and H. G. Greig in Electrofax- Direct Electrophotography Printing on Paper, RCA Review, December 1954, volume 15, No. 4, pages 469 to 484.

The zinc oxides which are suitable are those which are substantially electrically non-conductive in the dark. When exposed to light they should exhibit a surface photoconductivity or" a certain level in order to be of practical use for the purposes of this invention. In testing zinc oxides to determine their suitability, it is convenient to express the results of the measurements of the test as surface photoconductivity because substantially all of the light is absorbed in a thin layer at the surface of the zinc oxide. It has been found that the zinc oxide selected should have a surface photoconductivity of at least l0*9 ohm-1/square/watt/cm.2 when exposed to radiation having a wavelength of about 3900 A.

Typical zinc oxides suitable for this purpose include: 1) Florence Green Seal No. 8 marketed by the New Jersey Zinc Company, Palmerton, Pa.; (2) U.S.P. XIV marketed by Mallinckrodt Chemical Works, New York, NY.; (3) U.S.P. marketed by Eagle-Picher Sales Company, Philadelphia, Pa.; and (4) Cat. No. Z-53 marketed by Fischer Scientific Company, Division of Eimer and Amend Company, New York 14, N.Y.

After ball-milling to obtain a smooth uniform consistency, the mixture is coated on the surface of a paper web and dried. Any standard .coating technique may be used such as flowing, spraying, dipping, 'spin-coating, or brushing on.

The developer powder particles of the 'developer mix may be chosen from a large class of materials, for example: zinc, copper, carbon, sulphur, gum copal, 'gun sandarac, nylon, polystyrene, sealing wax and other natural or synthetic resins or mixtures thereof. rl`he developer powder particles may be coated with a thin layer of a material for the purpose of modifying the physical or electrical properties of the developer powder. It is preferred, however, to use a pigmented thermoplastic synthetic resin.

A preferred developer powder example may be prepared as follows: a mixture comprising 200 grams of 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs) marketed by the Pennsylvania Industrial Company, Clairton, Pa., and l2 grams of Carbon Black G marketed by the Eimer and Amend Co., New York, NY., are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short a time as possible. The melt is poured upon a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball-milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive triboelectric charge when mixed with iron powder.

An effective and practical method for bringing the properly charged developer powder particles to the surface to be developed is called magnetic brush development. The brush comprises the developer powder and the iron powder mixed therewith picked up on the end of a magnet. The magnet causes the powder particles to arrange themselves in the well known manner along the lines of force of the magnet to form the fibers of a brush.

The powder mixture is picked up by the magnet and the resultant brush is swept across the negative electrostatic latent image on the surface to be developed. When the brush passes over a charged area and the electrostatic attraction between the charged developer powder particles and the oppositely charged image areas is greater than the attraction between the developer powder and the iron powder, the developer powder is stripped from the brush and adheres quite strongly to the paper, forming the visible image. rlhe iron powder is retained in the brush by magnetic attraction. The developer mix is thus handled and brought into position by magnetic forces, while the visible image builds up under electrostatic forces. The selective effect is very denite.

The magnetic carrier particles of the developer mix may be chosen from a large class of powdered magnetically-attractable materials such as iron, steel, alloys of aluminum, nickel and cobalt and other magnetic materials.

A preferred carrier material for the developer mix of the example consists of arcoholized iron, that is, iron particles free from grease and other impurities soluble in alcohol. These iron particles are preferably relatively small in size, being in their largest dimension about 0.002 to 0.008 inch. Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat wider range of sizes up to about 0.001 to 0.020 inch. It is preferred to utilize a permanent bar magnet for providing the magnetic eld for maintaining the developer mix in a loose mass as in the magnetic brush. However, other structures may be used, such as electromagnets or other magnetic field producing means.

The exact steps actually employed in making a reverse print on the photoconductive zinc oxide layer sensitized with Rose Bengal are as follows: (1) A paper sheet is sensitized by giving it a blanket negative charge in the dark with a 6 kilovolt corona discharge source; (2) a photographic negative is placed on the sheet and exposed for 21/2 seconds to a 100 watt white tungsten lamp located 18 inches from the surface; (3) the photographic negative is remove.. and another blanket charge applied as in step (1); (4) the sheet now bearing a latent electro- -static image is exposed for about 1/10 second with a 100 watt-yellow lamp spaced 13 inches from the surface; and (5) a developer powder comprising the Piccolastic resin described. above, carrying a positive electrostatic charge, is applied to the surface, thereby forming a reverse visible image. The image is fixed to the photoconductive coating by applying heat thereto, for example, from an infrared lamp or other heat producing means.

What is claimed is:

1. A method of electrostatic printing comprising the steps of applying a first uniform electrostatic charge of one polarity to a surface of a photoconductive insulating layer, projecting a pattern of radiant energy to which said layer is sensitive to provide a second electrostatic image trostatic image, then applying a second uniform electrostatic charge of said one polarity to said surface, briefly flooding said surface with radiant energy to which said layer is positive to provide a' second` electrostatic image in reverse configuration with respect to first electrostatic image, `and applying an electrostatically charged developer substance to said surface whereby a reverse visible image is produced.

2. A method of electrostatic printing comprising the steps of producing a first uniform electrostatic charge of one polarity upon a surface of a photoconductive material which comprises a photoconductor dispersed in an electrically insulating vehicle, projecting a pattern of radiant energy to which said layer is sensitive upon said surface thereby producing a charge pattern substantially corresponding to said light image, then producing a second uniform electrostatic charge of saidone polarity upon the surface of said photoconductive material, briefly flooding said surface with radiant energy to which said layer is sensitive whereby the configuration of the charge pattern Vof said electrostatic image is reversed, and applying an electrostatically charged developer substance to said surface whereby a reverse visible image is produced.

3. A method of electrostatic printing comprising thev vto which said photoconductive material is sensitive whereby the configuration of the charge pattern of said electro static image is reversed, and applying an electrostatically charged developer substance to said surface whereby' a reverse visible image is produced.

l4. A method of electrostatic printing comprising the steps of passing a first negative coronadischarge over the surface of a photoconductive material which comprises a photoconductor dispersed in an electrically i11- sulating vehicle, projecting a pattern of radiant energy to which said photoconductive material is sensitive upon said surface thereby producing a charge pattern substantially corresponding vto said pattern of radiant energy, then passing a second negative corona discharge over the surface of said photoconductive material, uniformly flooding said surface with radiant energy to which'said `photoconductive material is sensitive whereby the configuration of the charge pattern of said electrostatic image is reversed, and rapplying an electrostatically charged .developer to 'said surface whereby a reverse visible image is produced. I

5.,A method of electrostatic printing for obtaining a reverse visible image comprising the steps of uniformly vcharging the surface of a photoconductive material lwith :formly flooding said photoconductive surface with radiant energy to which said photoconductive material is sensitive,`an'd depositing a nely divided electrostatically attractive material on said surface whereby Va reverse visible image is produced." l

6. An improved method of electrostatic printing wherein a reverse visible image is'obtained on a surface of a photoconductive insulating material disposed upon a nonconductive backing, which comprises in sequence the steps of a first charging of the surface of said material with a distributedV electrostatic charge of one polarity, eX- posing said surface to a radiant energy to which said photoconductive material is sensitive 'comprising a pattern of light and shadow to be recorded whereby said charge is reduced in the illuminated areas of said'surface leaving an electrostatic charge imageV thereon, a second charging of the surface of said exposed layer with a distributed electrostatic charge of said one polarity, briefly flooding said layer with uniform illumination to which photoconductive material is sensitive whereby said charge `image is reversed in configuration, and applying an elec- Y a reversed visible image is produced.

7. A method of electrostatic printing comprising the steps of projecting a light image upon a Vsurface of a photoconducting insulating layer disposed upon a nonconductive backing, said layer comprising a finely-divided photoconductive zinc oxide dispersed in an insulating f binder and said light image comprising a pattern of radiant energy to which said photoconductive' layer is sensitive then applying a uniform electrostatic charge to said surface, briefly flooding said charged Vsurface with radiant energy to which said photoconducting layer is sensitive, and applying' an electrostatically charged developer powder to said surface whereby a'reverse visible image -is produced.

References Cited in the ileof this patent j 'I UNITED STATES PATENTS 2,690,394

Carlson Sept. 28, 1954 2,741,959 Rheinfrank Apr. 17, 1956 2,756,676 Steinhilper July 31', 1956 2,764,693 Jacobs et al Sept. 25, 1956 2,808,328 Jacob Oct. .1, 1957 2,817,765 YHayford Dec. 24, 1957 "2,833,648 Walkup May 6, 1958 2,845,348 t Kallma'n .'ulyV 29, 1958 Y2,853,383 Keck v Sept. 23, 1958 `2,863,767 Vyve'rberg Dec. 9, 1958 2,914,403 y Sugarman1; Nov. 24, 1959 f v FOREIGN PATENTS j .521,415 e Canada Ian. 3 1, 1956 a negative corona source, exposing said surface to a ,pattern of radiant energy to which said'photoconductive material is sensitive,`then a second uniform charging of said exposed surface with a negative corona source, uni- 'j OTHERREFERENCES' j Wainer: Photographic Engineering, vol. 3,.No. 1,-pp.

' Dessauer etal.: Photographic Engineeringpvol.` 6, No.

'4, pp. 252-2155. Y Y

UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 2,979,403 April 112 1961 Edward C. Giaimo, Jr,D

It is hereby certified that error appears in the above numbered patent requiring correction and that Jthe said Letters Patent should read as 'corrected below Column lines 72 and '73v strike out "to provide a second electrostatic image trostatic image and insert instead upon said surface to provide a first electrostatic image m; column T, line 1, for "positive" read sensitive #ma SEA L) Attest:

EENESTW. SWIDEE Attesting Officer DAVID L. LADD Commissioner of Patents USCOMM-DC UNITED STATES PATENT OFFICE CERTIFICATE 0E CORRECTION Patent No. 2,979,403 April ll7 1961 Edward C. Giaimo, Jr.,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent, should read as "corrected below Column lines 72 and 73, strike out "to provide a y second electrostatic image trostatic imageH and insert instead upon said surface to provide a first electrostatic image ma column 7, line l, for "positive" read sensitive Signed and sealed this 12th day of September 1961 (SEAL) Attest:

ERNEST'W. SWIDER Attesting Officer I DAVID L. LADD Commissioner of Patents USCOMM-DC- 

1. A METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF APPLYING A FIRST UNIFORM ELECTROSTATIC CHANGE OF ONE POLARITY TO A SURFACE OF A PHOTOCONDUCTIVE INSULATING LAYER, PROJECTING A PATTERN OF RADIANT ENERGY TO WHICH SAID LAYER IS SENSITIVE TO PROVIDE A SECOND ELECTROSTATIC IMAGE TROSTATIC IMAGE, THEN APPLYING A SECOND UNIFORM ELECTROSTATIC CHANGE OF SAID ONE POLARITY TO SAID SURFACE, BRIEFLY FLOODING SAID SURFACE WITH RADIANT ENERGY TO WHICH SAID LAYER IS POSITIVE TO PROVIDE A SECOND ELECTROSTIC IMAGE IN REVERSE CONFIGURATION WITH RESPECT TO FIRST ELECTROSTATIC IMAGE, AND APPLYING AN ELECTROSTATICALLY CHARGED DEVELOPER SUBSTANCE TO SAID SURFACE WHEREBY A REVERSE VISIBLE IMAGE IS PRODUCED. 